If someone is tamei, does that person have to use the mikvah twice or once is enough? If you have to use it twice how is this done?
Firstly, the Rambam in הלכות מקוות פרק א says:
והטובל פעמיים במקוה, הרי זה מגונה.
"Somebody who immerses twice is reprehensible." So it's clear that you never have to immerse more than once.
Secondly, there's no obligation for men who are tamei to go to a Mikvah nowadays.
As a result, your question "does that person have to use the mikvah twice or once is enough" is based on a false premise.
The correct answer is: If someone is tamei he doesn't have to go to the Mikvah at all.